Conversion of aliphatic hydrocarbons to aromatic hydrocarbons



I l l l l l l United tat Patiit CONVERSION OF ALIPHATIC HYDROCARBONS' TAROMATIC HYDROCARBONS No DrawingI Application February 18, 1955 t SerialNo. 489,303

8 Claims. (Cl. 260673.5)

'Thisinvention relates to the conversion of aliphatic hydrocarbonscontaining at least six carbon atoms and containing a quaternary carbonatom, and more particu larly to the conversion of such hydrocarbons toaromatic hydrocarbons.

Various methods have been proposed and utilized for the dehydrogenationof organic compounds. Thus, ethylene is made by the dehydrogenation ofethane; butyl-- enes-land -2 by the dehydrogenation of n-butane;isobutylene by the dehydrogenation of isobutane; butadiene by thedehydrogenation of n-butane, and/or of the nbutenes; benzene by thedehydrogenation of cyclohexane; toluene by thedehydrogenation ofmethylcyclohexane; the xylenes by the dehydrogenation of thedimethylcyclohexanes, and the like. Also, it is known todehydroisomerize certain compounds, such as in the conversion ofmethylcyclopentane to benzene and of dimethylcyclopentane to toluene.These prior processes are usually high temperature pyrolysis operationsor are heterogeneous phase catalytic operations.

It is a principal object of the present invention to provide an improvedprocess for the conversion of a particular class of hydrocarbons. Morespecifically, a particular object of the invention is to provide aprocess for the dehydroisomerization of aliphatic hydrocarbonscontaining at least six carbon atoms and containing a quaternary carbonatom, to a product which contains at least one less quaternary carbonatom. A more specific object is to provide a process for thedehydroisomerization and cyclization of 2,4,4-trimethylpentene to anaromatic hydrocarbon. Another specific object is topro-' videa processfor the dehydroisomerization and cyclization of 2,2,5-trimethylhexane toan aromatic hydrocarbon. These objects will be more fully understood andothers will become apparent from the description of the invention.

l Now, in accordance with the present invention, it has been found thatan aliphatic hydrocarbon which contains at least six carbon atoms andcontains a quaternary carbon atom is converted, in the presence of asubstantial proportion of free iodine, at an elevated temperature to ahydrocarbon which contains at least one less quaternary carbon; atom.Furthermore, when there is only one quaternary carbon atom'in a chain ofat least five contiguous carbon atoms (at least four in addition 'to thequaterriarykarbon atom), 'the hydrocarbon reacts with" iodine at anelevated temperature to undergo isomerization and further may also becyclized to a cyclic hydro carbon, and particularly dehydrocyclized toan aromatic hydrocarbon. The at least four carbon atoms may bedistributed to provide onlytwo carbon atoms on either side of thequaternary carbon atom or so as to provlde a chain o f 'at' least"three' contiguous carbon atomson one side."

The invention is applicable particularly to aliphatic hydrocarbonswhichcontain only one quaternarv carbon atomin a chain of at least fivecontiguous carbon atoms fromthree to five contiguouscarbon atoms, asillustrated by 2,2,4-trimethylpent'ane, 2,4,4-trimcthylpentene-l, 2,2,3-trimethylpentane, 2,2,4-trimethylhexane, 3,3,4-trimethylhexane, 2,2,5-trimethylhexane and 2,4,4-trimethylhexane. v 1

When the hydrocarbon contains two quaternary carbon atoms in a chain ofat least'four carbon atoms the re-, action involves adehydroisomerization coupled with a further dehydroisomerization and/ordealkylation leading by cyclization to the formation of a chain ofsixnonquaternary carbon atoms in an aromatic ring.

The reaction of'the hydrocarbon and the iodine is suitably carried outby heating a mixture of them to a temperature sufiicifentlyelevated' toeitect a dehydrogenation reaction of the iodine with the hydrocarbon.The reaction is carried, out in vapor phase and can be effected bypassing the mixture of reactants. through a suitable heated reactionzone, which may be unobstructed, or it can be suitably provided withparticulate solid material to increase the heat transfer rate,intermingling of the reacant molecules, and the like. Such solidmaterial will usually be chemically inert toward the reactants under theconditions of the process, although it will be understood that materialscan be utilized which exert some desirable catalytic effect upon thereactants or on the products, provided the essential reaction of theinvention is not materially interfered with. The reaction is readilyeffected by suitably heating the hydrocarbon and iodine vaporsunderatmospheric pressure. However, pressures either lower than or higherthan atmospheric pressure can be utilized.

in the case of a given reactant more severe temperature: conditions canbe utilized while at the same time utilizing;

of the reaction zone. Other factors which are determina- Diluent vaporsof other substances can be admixed with the reactants in order to reducethe concentrations thereof at a given temperature and under a giventotal pressure. Other dehydrogenating agents, particularly such asoxygen, sulfur and the like, can be incorporated. in the reactantmixture provided they do not materially change the nature of thehydrocarbon product. Thus, oxygen can be added, either at a single pointin the flow, either ahead of, or in the reaction zone, or at a pluralityof points in the reaction zone. Such an oxidizing agent, of course, willreact with the hydrogen iodide to form water and liberate free iodine. I

In general, the reaction can be carried out at temperatures within abroad range of temperatures.v Thus, temperatures rangingpfrom 300 C. to600 C. are suitable, even though still lower and still highertemperatures are useful, particularly depending on the particularhydrocarbon reactant and the particular product desired. Particularlyuseful temperatures range from-about 350 C. to about 550 C. It will beunderstood, of course, that a short residence time in the reaction zone.

The residence time of the hydrocarbons under the reaction conditions isin general'relatively small, ranging from a fraction of a second, e.g.,0.01 second, up to as high as several minutes, e.g., 3-5, with fromabout 0.1'

to 60 seconds being satisfactory in the usual cases. This, as alreadyindicated, depends in part on the temperature tive of the residence timeof the hydrocarbon include the molar ratio of iodine to hydrocarbon, thenature of the desired product, and the particular hydrocarbon reactantutilized.

hydro'carbomthe proportion of iodine used'in the reac-' tion depends onthe extentof the dehydrogenation ofv theindividual molecules, whether.to form a mono-olefin. and the quaternary carbon atom -ieioinedtoachainof from a saturated hydrocarbon, a diolefin from asaturated:

hydrocarbon, a diolefin from a mono-olefinic hydrocarbon, an aromatichydrocarbon from a saturated aliphatic hydrocarbon, or an aromatichydrocarbon from an unsaturated aliphatic hydrocarbon, etc., as well asthe percentage conversion of the hydrocarbon which is desired in asingle passage through the reaction zone. In general, from about 0.05 toabout 3 or 4 equivalents of iodine can be utilized for each equivalentof hydrogen to be removed to yield the desired hydrocarbon product,although it is usually preferable to utilize at least about 0.2equivalent and amounts corresponding to no more than about oneequivalent give satisfactory results.

Although the nature of the isomerization reaction which occurs is notentirely understood, the evidence indicates that there is a cleavage ofthe bond joining the quaternary carbon atom to one of the carbon atomsand the formation of a new C-to-C linkage between the carbon atom fromwhich the quaternary carbon atom is cleaved and one of the other carbonatoms bonded directly to the quaternary carbon atom. The presence of achain of at least two, and particularly three, non-quaternary carbonatoms attached to the quaternary carbon atom appears to be particularlyfavorable to the isomerization. For example, the results from thereaction of 2,2,4-trimethylpentane suggest that during the course of thereaction the 2,2,4-trimethyl-C -hydrocarbon-chain structure isisomerized to a 2,S-dimethyl-C -hydrocarbon structure; similarly, in thecase of 2,4,4-trimethylpentene-l. In the case of 2,2-dimethylpentane,the isomerization gives a Z-methyl-C -hydmcarbon structure. Similarly,isomerization of 3,3-dimethylpentane yields a 3-methyl-C -hydrocarbonstructure. Dehydroaromatization of this structure gives toluene. Asstated hereinbefore, those hydrocarbons which give rise by isomerizationto a chain of six non-quaternary carbon atoms are recoverable at leastto a substantial extent in the form of the cyclized and dehydrogenatedsix-carbon ring hydrocarbons, namely; aromatic hydrocarbons.

Having described the general features of the invention and variousfactors which are involved in its practice, details thereof will beillustrated by the following illustrative examples, which are not to beconsidered as limiting thereon.

Example I A vaporous mixture of iodine of 2,2,4-trimethylpentane in amolar ratio of iodine/hydrocarbon of 0.31 was passed through an emptyVycor reactor (4 cm. inside diameter; 811 cc. volume) at a temperatureof 444 C. and one atmosphere pressure at a nominal residence time, basedon total molar input at reaction temperature, of 26 seconds. The productstream was collected and analyzed to determine the nature andproportions of the components thereof. Based on the analysis, 34% of thehydrocarbon feed was reacted. The reacted iodine occurred in the productstream essentially as hydrogen iodide, with a very small proportion inthe form of C to C iodides, while the hydrogen was accounted foressentially entirely as hydrocarbon or hydrogen iodide. For each 100moles of hydrocarbon reacted, 30.1 were accounted for as c -hydrocarbons(21.8 as C olefin, principally 2,4,4-trimethylpentene, 7.8 as p-xylene,0.4 as m-xylene and 0.07 as o-xylene) and 68.5 moles were accounted foras C.,-hydrocarbons, isobutane and isobutene.

Since the formed 2,4,4-trimethylpentene is a precursor for aromatics, asseen from. Example III, it is clear that in this example the limitedproportion of free iodine used was limiting on the yield of aromatics.Although cleavage to two iso-C fragments was the principal result fromthe reaction under the conditions of Example I, the iso-C products arereadily convertible again to the 2,2,4- trimethyl-C structure, so thattheir formation does not represent any net loss.

' 4 Example II A vaporous mixture of iodine and 2,2,5-trimethylhexane ina molar ratio of iodine/hydrocarbon of 1.0 was passed through thereactor of Example I at a temperature of about 475 C. (462488 C., inletand outlet, respectively) and one atmosphere pressure at a nominalresidence time of 8 seconds. Based on analysis of the product streamfrom the reactor, 69% of the hydrocarbon reacted and of the iodinereacted. The iodine was essentially entirely present as hydrogen iodideand free iodine, and the reaction was clean with the essential absenceof coke formation. For each moles of the hydrocarbon reacted, theproduct contained about 67 moles of C -monoalkene and of the order of 3moles of C -alkadiene (there was practically no cycloalkane orcycloalkene), a portion of aromatic hydrocarbon, all being C or lower,there being 4.2 moles of m-xylene, about 15 moles of iso-C -hydrocarbonsand about 17 moles of iso-C -hydrocarbons, being essentially isopentane,isopentene, isobutane and isobutene.

Example III A vaporous mixture of iodine and diisobutylene (21%2,4,4-trimethylpentene-2 and 79% 2,4,4-trimethylpentene-l) in a molarratio of iodine/hydrocarbon of 0.3 was passed through the reactor ofExample I at a temperature of about 440 C. (425454 C., inlet and outlet,respectively) and one atmosphere pressure at a nominal residence time of10 seconds. Based on analysis of the product stream from the reactor,32% of the hydrocarbon and 75% of the iodine reacted. The iodine waspresent as hydrogen iodide and free iodine, together with a small amountof C and C iodides, and the reaction was clean with the essentialabsence of coke formation. On the basis of 100 moles of the hydrocarbonreacted, the product stream contained: 28 moles of C l-I (principally2,2,4-trimethylpentane), 9 moles of C diolefin, 35 moles of paraxylene,16 moles of isobutane, 28 moles of isobutylene, 1 mole of methane, and0.5 mole of hydrogen.

The isobutane is convertible to isobutene, which, together with thatformed in the reaction, is convertible to diisobutylene. This, togetherwith the non-aromatic C -hydrocarbons recoverable from the reactionproduct stream is suitably recycled with iodine to the reaction zone forthe production of further amounts of aromatic (p-xylene).

We claim as our invention:

1. A process for the conversion of an aliphatic hydro carbon whichcontains a chain of at least four carbon atoms and in which the sum of(l) the total number of carbon atoms in the chain plus (2) the number ofquaternary carbon atoms in the chain is at least six and the longestchain contains no more than five contiguous non-quaternary carbon atomsinto an aromatic hydrocarbon containing no quaternary carbon atom, whichcomprises subjecting a mixture of said hydrocarbon and at least about0.3 mole of iodine per mole of hydrocarbon in vapor phase for a time offrom 0.01 second to five minutes to a temperature of at least 300 C. toeffect a bond cleavage in the hydrocarbon in the presence of free iodineand convert a quaternary carbon atom of the hydrocarbon to anon-quaternary carbon atom, and recovering an aromatic hydrocarbonproduct.

2. A process in accordance with claim 1, wherein the hydrocarbon is analkane.

3. A process in accordance with claim 1, wherein the hydrocarbon is analltene.

4. A process in accordance with claim 1, wherein the quaternary carbonatom of the hydrocarbon is in a chain of only five contiguous carbonatoms.

5. A process in accordance with claim 1, wherein the quaternary carbonatom of the hydrocarbon is in a chain of only six contiguous carbonatoms.

2,880,262 5 6 n 6. A process in accordance with claim 4, wherein the2,315,499 Cantzler et al. Apr. 6, i943 hydrocarbon is2,2,4-trimethylpentane. 2,492,844 Condon Dec. 27, 1949 7. A process inaccordance with claim 4, wherein the OTHER REFERENCES hydrocarbon is2,4,4-trimethylpentene-1.

8. A process in accordance with claim 5, wherein the 5 ChemlcalAbstracts 25 (1931) 9,

Bairstow et al.: Journal of the Chemical Society h drocarb n i 2,2,5-tr'th lh x y o 5 me y e m 1933 p. 1158.

References Cited in the file of this Patet Rossini et al.: Hydrocarbonfrom Petroleum (1953),

UNITED STATES PATENTS page 406, Reinhold Publishing Corp., New York.

1,925,421 Van Peski Sept. 5, 1933

1. A PROCESS FOR THE CONVERSION OF AN ALIPHATIC HYDROCARBON WHICHCONTAINS A CHAIN OF AT LEAST FOUR CARBON ATOMS AND IN WHICH THE SUM OF(1) THE TOTAL NUMBER OF CARBON ATOMS IN THE CHAIN PLUS (2) THE NUMBER OFQUATERNARY CARBON ATOMS IN THE CHAIN IS AT LEAST SIX AND THE LONGESTCHAIN CONTAINS NO MORE THAN FIVE CONTIGUOUS NON-QUATERNARY CARBON ATOMSINTO AN AROMATIC HYDROCARBON CONTAINING NO QUATERNARY CARBON ATOM, WHICHCOMPRISES SUBJECTING A MIXTURE OF SAID HYDROCARBON AND AT LEAST ABOUT0.3 MOLE OF IODINE PER MOLE OF HYDROCARBON IN VAPOR PHASE FOR A TIME OFFROM 0.01 SECOND TO FIVE MINUTES TO A TEMPERATURE OF AT LEAST 300*C. TOEFFECT A BOND CLEAVAGE IN THE HYDROCARBON IN THE PRESENCE OF FREE IODINEAND CONVERT A QUATERNARY CARBON ATOM OF THE HYDROCARBON TO ANON-QUATERNARY CARBON ATOM, AND RECOVERING AN AROMATIC HYDROCARBONPRODUCT.